Altitude control mechanism for carburetors



E. DODSON April 22, 1941.

ALTITUDE CONTROL MECHANISM FOR CARBURETORS Filed July 5, 1939 2 Sheets-Sheet 1 P 1941- E. DODSON 2,239,553

ALTITUDE CONTROL MECHANISM FOR CARBURETORS Filed m 5, 19:9 '2 She ets-Sheet 2 Patented Apr. 22, 1941 ALTITUDE CONTROL MECHANISM FOR CARBURETORS Edward Dodson, Putney Heath, London, England Application my 5, 1939, Serial No. 282,938 In Great'Britain July 14, 1938 Claims.

This invention relates to altitude control means for carburetors for internal combustion engines, and particularly aero engines.

, It is well known that, owing to the decrease in density of the air with increase in altitude, carburetors for aero engines supply a smaller proportion by weight of air to fuel with increase in altitude. Carburetors for such engines have, therefore, been fitted with a control for correcting the fuel-air ratio with change in altitude so as to tend to maintain this ratio approximately constant irrespective of altitude. It is known that in an uncorrected carburetor the error in the fuel-air ratio with change in altitude arises, primarily from the. fuel passing into the Jet being subjected to a head which varies according to the square root of the atmospheric pressure and not directly proportional to such atmospheric pressure,

I have conceived the idea of effecting the necessary correction by using aventuri with a branch pipe from its throat. When the pressure in such a .venturi is low enough to produce a critical velocity at its throat, the pressure in the branch pipe is unaffected by variations of suction applied at the outlet end of the venturi, but is affected .by variations of pressure at the venturi intake. Such a critical velocity occurs when the pressure in the throat is reduced to approximately half the initial pressure (1. e. the pressure at the intake. When a venturi is operating under such critical velocity conditions the pressure in the branch pipe varies according to thesquare root of the density and pressure of the air at the Venturi intake. Now since, for the reason previously xplained, the error in mixture strength arising fromthe pressure of fuel varies according to the square root of atmospheric pressure and since the pressure in the branch pipe of the venturi also varies as the square root of atmospheric pressure at the venturi intake, it follows that the branch pipe pressure can be used to correct the inherent error in the normal carburetor systems in a manner which will be hereinafter explained.

It will be understood that in carburetors the fuel is caused to flow into the carburetor choke tube or venturi by reason of the pressure there being lower than the pressure in the fuel chamber, and so, by sealing the carburetor fuel chamber and connecting it to the branch pipe of the venturi operating under critical velocity conditions, the pressure in the air space above the fuel in said chamber will vary in the required manner and will correct the effect of change in altitude on the flow of fuel to the jet or jets in the carburetor.

In practice, however, if the pressure in the branch pipe is applied directly to the fuel in the floa't chamber, the pressure in the float chamber would be so small thatfuel would not be induced to flow into the carburetor choke and, on the contrary, air might be drawn into-the jet in a reversedirection. The object of this invention, therefore, is to provide a practical means of compensating for variation in altitude, making use of the aforesaid variations of pressure in the branch pipe and yet nsuring that the pressure sure and its outlet open to said source of suction valve b and arranged to produce a critical velocity at its throat, a sealed casing containing a pressure sensitive device, a branch pipe communicating at its one end with the throat of said venturi and at its other end with the said sealed casin on one side of the pressure sensitive device, and a passage communicating the casing on the other side of the pressure sensitive device with the float chamber.

In the accompanying drawings examples of construction are illustrated and in these- Fig. 1 is a diagram of one form of construction,

Fig. 2 is a sectional elevation of a modified form of the sealed casing shown in Fig. 1, and

Fig. 3 is a section on the line 3-3 in Fi 2.

In the embodiment illustrated inxFig. 1, the inventionis shown as applied toa conventional type of carburetor having a floatv chamber a, a float a a needle valve 0. for maintaining the fuel level constant and a jet or jets a in the induction manifold I) provided with a throttle The .venturi c is formed in a by-pass passage communicating at one end c with the intake 1; of the induction manifold-belowthe r jet and at its other end c with the induction pipe above, the throttle valve. Thus the intake of the venturi c is open to atmospheric pressure while its outlet is connected to the suction in the induction manifold. At the throat of the venturi a branch pipe d is formed which is connected to a diaphragm casing, on one side of the diaphragm therein, while this casing is connected on the opposite side of the diaphragm to the float chamber above the'level of the fuel therein.

In the arrangement illustrated where two diaphragms e e are provided the branch pipe d from the venturi is divided to form three conduitsd .d.d=connectedtoadiaphragmcasinge and communicating respectively one al with the top of the casing, the second d with the centre of the casing and the third 41 with the bottom ofthecasing. Inthiscasingetwodiaphragms e, c are arranged across the casing between the parts with which the said three conduits d d, d communicate,.dividlng the casing into chambers e, e and e. The conduit d extends into the chamber e and is formed with an outlet port :1 controlled by a valve d. In the chamber e a fourth conduit f is arranged, which conduit communicates at one end with the branch 0 of the by-pass passage and contains at its other end an inlet port 1 controlled by an inlet valve F. A fifth conduit g extends between the diaphragm casing e and the float chamber and communicates the chamber e to the float chamber.

The diaphragm e is larger than the diaphragm 20 e and those diaphragms are connected to each other by the member e. Owing to the smaller size of the diaphragm e the total pressure acting on-this will beless than that on thediaphragm e Assuming now that the parts are at any-instant in the position shown in Fig. 1, in

which both valves are closed, and that the pres-.

sure in the pipe at is suddenly decreased, this decrease in pressure will be communicated to the chamber e, e through the pipes (F, d but it will not be communicated to the chamber e through the pipe :1 because the valve (1 is closed. Now owing tothe diil'erence in size of the diaphragms e, e" and to the fact heneinbefore explained that the diaphragm 4: will be subject to a greater total pressure than the diaphragm e", the diaphragm c will override that marked e, and these diaphra ms will be deflected upward. In doing so, the diaphragm e will open the valve d while maintaining the valve 1 on its seat. Thus the chamber e will be opened to the pipe 11 and the pressure in the chamber will be reduced until the diaphragms e e return to the position shown in Fig. 2, allowing the valve d? to close, by which time the pressure in the chamber e will have been reduced by an amount corresponding to that in the chamber a, e. A simflar action will take place if the pressure in d is increased instead of reduced, except that in this case the diaphragm will be deflected downward and will open the valve f, while leaving the valve (1 closed so that the air at a higher pressure in the pipe I will flow into the chamber e until the pressure in this chamber is increased by an amount corresponding to the increase in the chamber e 0 It will thus be seen that an increase or decrease of pressure in the chamber e e will cause movement of the diaphragms and on opening of the valves 1' or d the pressure in the chamber e will be increased or decreased as aforesaid in correspondence with the increase or decrease in the chamber e e and when the requisite pressure in the chamber e is attained the diaphra ms will return and close the valves.

It has been stated that the pressure in the pipe (I 65 is unaffected .by variations of pressure at the Venturi intake and since the suction intake communicates with the atmospheric pressure at the intake of the induction manifold, it will be clear that the pressure in the pipe it will vary with variation of atmospheric pressure and since the pressure in the chamber e will be varied in correspondencewith the pressure in the pipe d, it follows that by communicating the chamber e to the float chamber through the pipe 0 the pres- 75 sureintheairspaceabovethefuelinthefloat chamberwiilvaryintherequlredmannerand will correct the eifect of change in altitude on the flow of fuel to the carburetor jets.

A spring e is provided between the top of the maindiaphragm e andapressurepiecee'on top of the casing. The pressure piece is slidable up and down to enable the loading of the spring to be varied by a lever e' on the outside which may be connected to any suitable mechanism not shown for manually adjusting and maintaining the presure piece in any desired position.

It will be understood that although any change in pressure in the branch pipe (1 will, owing to the opening of the valve d or F produce an equal change in the pressure in the float chamber a, the pressure in the float chamber will always exceed that in the branch pipe by an amount depending on the difference in area of the diaphragms e e and the loading of the spring c", with the result that there is no danger.of air being drawn by suction from the carburetor choke into the float chamber.

A modified form of the diaphragm casing illustrated diagrammatically in Fig. 1 is shown in Figs. 2 and 3, in which again like references are used to indicate like parts. As shown in these two figures the conduits d and f are arranged side-by-side and the valves (1 and P are controlled by. a floating lever mechanism comprising the lever h pivoted at h to the interconnection e between the two diaphragms and carrying at its other end a central pivot 11.

v on which is mounted a sleeve 71. slidablycarried in the diaphragm casing and containing a spring h. The lever h is also formed with universal joints 11 h, one at each side of the pivot 71. for connecting the lever to rods h", h operatively connected to the valves d, 1'. Thus when the diaphragms are in the position shown the spring in tends to maintain both the valves seated. Movement of the diaphragms e a upwardly acts to rock the floating lever h about the universal joint h as a fulcrum to open the valve d while the movement of the diaphragms downwardly acts to rock the lever it about the universal joint 72 as a fulcrum to open the valve P. In this construction the conduits d (1 of Fig. 1 are constituted by a by-pass passage i in Fig. 3 bifurcated at i and communicating with outlet ports 1", i respectively above and below the diaphragms as indicated in chain dotted lines in Fig. 2. In Fig. 2 9 indicates an adaptor to which is connected the conduit 0 for connecting the diaphragm casing to the carburetor float chamber. As shown in addition to the spring e acting on the main diaphragm e a spring e" is provided acting on the secondary diaphragm e and each of these springs e" and e" is adjustable by the devices respectively marked e", e".

The diaphragms e e constitute pressure sensitive devices and it will be understood that instead of the diaphragm form of such devices, other forms thereof such as pistons or capsules may be used instead.

What I claim as my invention and desire to secure by Letters Patent is:

l. Altitude control means for carburetors comprising a source of suction, a venturi having its intake open to atmospheric pressure, and its outlet communicating with the said source of suction and arranged to produce a critical velocity at the Venturi throat, a sealed carburetor float chamber, a branch pipe communicating at its one end with the throat of said venturi,

a sealed casing, a pressure sensitive device in said casing, said branch pipe being connected to said casing so as to communicate with branch pipe inlets on both sides of the said device, an atmospheric pressure pipe connected to said casing and communicating with an atmospheric inlet on one side of the said device, a valve controlling the branch pipe inlet which is on the same side of the device as the atmospheric inlet and a valve controlling the atmospheric inlet, means operatively connecting said valves with said device whereby when the device is in a predetermined position both valves are closed but when the device is moved from said position in one direction one valve is opened and when the device is moved from said position in the other direction theother valve is opened, a pipe in the carburetor float chamber connected to said casing and communicating with its interior on the same side as the atmospheric inlet and loading means for the pressure sensitive device to cause the pressure in the float chamber pipe to exceed that in the branch pipe.

2. Altitude control means for carburetors comprising a source of suction, a venturi having its,

intake open to atmospheric pressure and its outlet communicating with the said source of suction, and arranged to produce a critical velocity at the venturi throat, a sealed carburetor float chamber, a branch pipe communicating at its one end with the throat of said venturi, a sealed casing, two pressure sensitive devices in said casing, said branch pipe being connected to said casing so as to communicate with branch pipe inlets on both sides of each of the said devices, an atmospheric pressure pipe connected to said casing and communicating with an atmospheric inlet on one side of one of the said devices, a valve controlling the branch pipe inlet which is on the same side of the device as the atmospheric inlet and a valve controlling the atmospheric inlet, means operatively connecting said valves with said devices whereby when the devices are in a predetermined position both valves are closed but when the device is moved from said position in one direction one valve is opened and when the device is moved from said position in the other direction the other valve is opened, a pipe in the carburetor float chamber connected to said casing and communicating with its interior on the same side as the atmospheric inlet and loading means for the pressure sensitive devices to cause the pressure in the float chamber pipe to exceed that in the branch pipe.

3. Altitude control means for carburetors comprising a venturi having its intake open to atmos pheric pressure, a source of suction connected to the outlet of said venturi, a sealed carburetor float chamber, a branch pipe communicating at its one end with the throat o1 said venturi, a

sealed casing, a pressure sensitive device in said,

casing, said branch pipe being connected to said casing so as to communicate with branch pipe inlets on both sides oi the said device. an atmospheric pressure pipe connected to said casing and communicating with an atmospheric inlet on one side of the said device, a valve controlling the branch pipe inlet which is on the same side as the atmospheric inlet and a valve controlling the atmospheric inlet, means operatively connecting said valves with said device whereby when the device is in a predetermined position both valves are closed but when the device is moved from said position in one direction one valve is opened and when the device is moved from said position in the other direction the other valve is opened, a pipe in the carburetor float chamber connected to said casing and communicating with its interior on the same side as the atmospheric inlet and loading means for the pressure sensitive device to cause the pressure in the float chamber pipe to exceed that in the branch pipe.

4. Altitude control means for carburetors comprising a source of suction, a venturi having its intake open to atmospheric pressure and its outlet communicating with the suction source and arranged to produce a critical velocity at the Venturi throat, a sealed carburetor float chamber, a branch pipe communicating at its one end with the throat of said venturi, a sealed casing, at least one pressure sensitive device in said casing, said branch pipe being connected to said casing so as to communicate with branch pipe inlets on each side of the said device, an atmospheric pressure pipe connected to said casing and communicating with an atmospheric inlet on one side of the said device, valves controlling the atmospheric inlet and the branch pipe inlet on the same side of the device as said atmospheric inlet, means operatively connecting said valves with said device whereby when the device is in a predetermined position both valves are closed but when the device is moved from said position in one direction one valve is opened and when the device is moved from said position in the other direction the other valve is opened, a pipe in the carburetor float chamber connected to said casing and communicating with its interior on the same side as the atmospheric inlet, loading means for the pressure sensitive device to cause the pressure in the float chamber pipe to exceed that in the branch pipe, and means for adjusting the loading of the said pressure sensitive device.

5. The combination with a carburetor having a sealed fuel chamber of altitude control means, comprising a source of suction, a venturi having its intake open to atmospheric pressure and its outlet connected to the source of suction, said source of suction being effective to produce a critical velocity at the throat of the venturi, a sealed casingdivided into two non-communicating portions, a branch pipe connecting one portion of the casing with the throat of the venturi, a pressure sensitive device in the casing which is responsive to changes in pressure in said portion of the casing, a conduit connecting the other portion of the casing with the fuel chamber, inlets for admitting atmospheric and sub-atmospheric pressure to said other portion of the easing, a valve associated with each of said inlets, means operating normally to maintain both valves closed, operative connections between the pressure sensitive device and the valves for effecting diflferential opening or the valves on change of pressure in the branch pipe, in the sense to open the atmospheric pressure inlet when said pressure rises and to open the subatmospheric pressure inlet when said pressure falls, thereby producing fluctuations in the pressure in the fuel chamber corresponding to those in the branch pipe, and means for loading the pressure sensitive device so that the pressure in the fuel chamber exceeds that in the branch pipe.

EDWARD DODSON- 

